1. Field of the Invention
This invention relates to gain stages and, more specifically, to a programmable switched-capacitor operational amplifier gain stage.
2. Description of the Prior Art
Gain stages are well known in the prior art. A gain stage produces an amplified output signal, which can be many times greater than the input signal. In general, the amplification, or gain, depends upon the ratio of the passive elements in the gain stage. Typically, resistors are used as the passive elements.
One common type of prior art gain stage is the operational amplifier gain stage. As shown in FIG. 1, this particular type of gain stage 10 includes an operational amplifier 11, a first passive element 12 located in the feedback loop 13, which provides a connection between the output lead 14 and the inverting input lead 15 of the operational amplifier, and a second passive element 16 located between the input terminal 17 and the inverting input lead 15. The gain of this operational amplifier gain stage is equal to the negative of the ratio of the resistance of the feedback resistor 12 to the resistance resistor 16: EQU G.sub.10 =-R.sub.12 /R.sub.16 ( 1)
where
G.sub.10 is the gain of gain stage 10; PA1 R.sub.12 is the resistance of resistor 12; and PA1 R.sub.16 is the resistance of the resistor 16. PA1 G.sub.100 is the gain of programmable gain stage 100; PA1 C.sub.17 is the effective capacitance of capacitor array 17; and PA1 C.sub.19 is the capacitance of feedback capacitor 19.
Unfortunately, offset voltages cause these prior art operational amplifier gain stages to provide a range of output signals less than that theoretically available from ideal operational amplifier gain stages. An offset voltage is a voltage that appears when the noninverting input lead of an operational amplifier is connected to ground and the inverting input lead is connected to the output lead of the operational amplifier. Theoretically, these offset voltages should not be produced, but because of inevitable component mismatches during fabrication, all operational amplifiers produce offset voltages--even when there is no voltage applied to the amplifiers. The value of the offset voltage cannot be determined before the fabrication of the amplifier. Furthermore, the offset voltage varies between devices and with time and temperature.
Offset voltages are undesirable. They limit the dynamic range of output signals provided by the operational amplifier. As is well known, operational amplifiers have an active region in which the output signal is proportional to the input signal. Beyond this active region, the operational amplifiers are saturated, that is, they produce the same output signal regardless of the input signal. Offset voltages effectively diminish the active region since they are a part of the input signal. As a result, the dynamic range of output signals provided by the operational amplifier is less than that theoretically available from ideal amplifiers.
Switched capacitors have been used as the passive elements in operational amplifier gain stages, for example, by Hosticka et al., "MOS Sampled Data Recursive Filters Using Switched Capacitor Integrators", IEEE Journal of Solid-State Circuits, Vol. SC-12, No. 6, pages 600-608, December 1977, which is hereby incorporated by reference.
One problem associated with the use of a switched capacitor in the feedback loop is the generation of another form of offset voltage. Whenever MOS switches are turned off or on, a clock feedthrough voltage appears at the output of the operational amplifiers. This clock-induced feedthrough voltage appears as a result of inevitable gate-to-drain or gate-to-source capacitance mismatches. Ultimately, this feedthrough voltage manifests itself as an offset voltage.
Either increasing the value of the feedback capacitor or reducing the size of the resetting MOS switch reduces the clock-induced feedthrough offset voltage. However, both of these methods are unsatisfactory. Both methods increase the RC time constant of the circuit. As a result, the settling time of the operational amplifier is restricted. In addition, the first method requires a large silicon area. If the voltage gain is to be 48 dB and the feedback capacitance 10 pF, the input capacitance would have to be 2512 pF--much too large for use in practical integrated circuits.
Other prior art operational amplifier gain stages provide a programmable gain. Such prior art operational amplifier programmable gain stages are disclosed, for example, in U.S. patent application, Ser. No. 249,775 (now U.S. Pat. No. 4,422,155) on an invention of Amir; U.S. patent application, Ser. No. 292,870 (now U.S. Pat. No. 4,438,354), on an invention of Haque, et al.; U.S. patent application, Ser. No. 310,160 (now U.S. Pat. No. 4,441,082), on an invention of Haque; U.S. patent application, Ser. No. 316,183 (now U.S. Pat. No. 4,470,126), on an invention of Haque; and U.S. patent application, Ser. No. 381,807, filed May 25, 1982, on an invention of Amir, et al., each of which is assigned to American Microsystems, Inc., the assignee of the present invention, and each of which are hereby incorporated by reference.
FIG. 2 depicts one such operational amplifier programmable gain stage utilizing switches capacitors as the passive elements. Programmable operational amplifier gain stage 100 of FIG. 2 provides an array 17 of N capacitors 17-1 through 17-N; each of the capacitors 17-1 through 17-N being switchably connected via a pair of switches 17-1a, 17-1b through 17-Na, and 17-Nb between the inverting input lead 70 of operational amplifier 18 and the input terminal 11 of gain stage 100. Selected ones of the N capacitors 17-1 through 17-N are connected between the inverting input lead 70 of operational amplifier 18 and input terminal 11 in order to select the effective capacitance of the capacitor array 17. As a result, the desired gain of gain stage 100 is selected as follows: EQU G.sub.100 =-C.sub.17 /C.sub.19, (2)
where
U.S. patent application, Ser. No. 292,870 on an invention of Haque, et al., entitled "Monolithic Programmable Gain--Integrator Stage" (now U.S. Pat. No. 4,438,354) which is assigned to American Microsystems, Inc, the assignee of this invention, and which is hereby incorporated by reference, describes a circuit that eliminates the effects of the offset voltages produced by the gain stages. The Haque circuit includes an integrator stage, which first integrates the positive of the offset voltages produced by the gain stages and then integrates the negative of the offset voltages. The result is an integrated output voltage that is free of the effects of the offset voltages from the gain stages. The problem with this device is that it relies on an integrator stage to eliminate the offset voltages.